TipTap editor, version history, search, attachments — and a cleanup

The app is meant to run in daily private use now, so this pass was mostly
about the ways it could quietly lose data.

Reliability and security:
- the server no longer invents a JWT_SECRET or ENCRYPTION_KEY when one is
  missing; it refuses to start. The old fallback silently made every stored
  note unreadable after a restart.
- notes are encrypted with AES-256-GCM (v2:); existing CBC bodies still read.
- login is rate limited and compares against a dummy hash when the user does
  not exist, so timing does not reveal who is registered.
- a server response issued before a local save is never adopted. This had
  already eaten drawing strokes and note links; sync.hasPendingWrites() now
  guards every path that takes a body from the server.

Editor and features:
- TipTap replaces the contenteditable editor: tables, task lists, and links
  to other notes and notebooks, with a backlinks bar on the target.
- content format v2 ({v:2,doc}); v1 {html,tasks} bodies migrate on open.
- version history: the state before each write is snapshotted, coalesced
  into 5-minute windows, 50 versions or 90 days per note.
- full-text search runs in the client, because the server cannot read
  encrypted bodies and a server index would defeat the encryption.
- image attachments are stored by SHA-256 and deduplicated; the client
  fetches them authenticated and caches the blob, so they work offline.
- drawings are vector strokes in a fixed 1200x1600 page space and now look
  the same on every screen (0.0-0.2% deviation across four widths).
- login is by username; existing email accounts migrate automatically.

Operations:
- data lives on host bind mounts, not Docker volumes, so Proxmox Backup
  covers it. Back up .env with it — without ENCRYPTION_KEY the dump is
  unreadable.

Structure:
- NoteApp.jsx is gone. App.jsx now holds only where the user is; note data,
  search, layout, history and drag each live behind a hook, and the four
  panes are stateless.
- the whole data layer is TypeScript, with 81 unit tests concentrated on
  the places that have actually broken.
- fixed along the way: the browser back button had a dead press, because
  restoring a history entry pushed a duplicate one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/src/ui/theme.ts

@@ -0,0 +1,80 @@
+/**
+ * The application palette.
+ *
+ * Colours live here rather than inline so that a change to, say, the danger red
+ * happens in one place. Modal components still carry small local palettes of
+ * their own — their key names mean different things (a modal's `bg` is a raised
+ * surface, the app's `bg` is the page behind it), so they are deliberately not
+ * folded in here.
+ */
+
+export interface Theme {
+  bg: string;
+  sidebar: string;
+  sidebarActive: string;
+  card: string;
+  toolbar: string;
+  border: string;
+  borderLight: string;
+  text: string;
+  textSec: string;
+  muted: string;
+  faint: string;
+  accent: string;
+  accentSoft: string;
+  accentText: string;
+  danger: string;
+  success: string;
+  warning: string;
+  inputBg: string;
+  dashBg: string;
+}
+
+const DARK: Theme = {
+  bg: "#0d1117",
+  sidebar: "#010409",
+  sidebarActive: "#161b22",
+  card: "#161b22",
+  toolbar: "#161b22",
+  border: "#30363d",
+  borderLight: "#21262d",
+  text: "#e6edf3",
+  textSec: "#c9d1d9",
+  muted: "#8b949e",
+  faint: "#484f58",
+  accent: "#388bfd",
+  accentSoft: "#388bfd22",
+  accentText: "#58a6ff",
+  danger: "#f85149",
+  success: "#3fb950",
+  warning: "#d29922",
+  inputBg: "#0d1117",
+  dashBg: "#0d1117",
+};
+
+const LIGHT: Theme = {
+  bg: "#f6f8fa",
+  sidebar: "#ffffff",
+  sidebarActive: "#eef2ff",
+  card: "#ffffff",
+  toolbar: "#ffffff",
+  border: "#d0d7de",
+  borderLight: "#e5e7eb",
+  text: "#1f2328",
+  textSec: "#424a53",
+  muted: "#656d76",
+  faint: "#afb8c1",
+  accent: "#0969da",
+  accentSoft: "#0969da18",
+  accentText: "#0550ae",
+  danger: "#cf222e",
+  success: "#1a7f37",
+  warning: "#9a6700",
+  inputBg: "#f6f8fa",
+  dashBg: "#f0f2f5",
+};
+
+export const makeTheme = (dark: boolean): Theme => (dark ? DARK : LIGHT);
+
+export const FONT_UI = "'DM Sans',sans-serif";
+export const FONT_SERIF = "'Newsreader',Georgia,serif";